• Inverted for the 3D chemical composition and thermal state of the lower mantle based on seismic tomography and mineral elasticity data
• Found that velocity heterogeneities in the upper lower mantle mainly result from thermal anomalies, whereas those in the lowermost mantle mainly result from compositional or phase variations
• Found that LLSVPs have ∼500 K higher temperature, higher bridgmanite and iron content than the ambient mantle, supporting the origin from an ancient basal magma ocean